DSL technology is a high speed transmission technology for data transmission via telephone twist pair, namely Unshielded Twist Pair (UTP), including Asymmetrical Digital Subscriber Line (ADSL), Very-high-bit-rate Digital Subscriber Line (VDSL), Digital Subscriber Line based on Integrated Services Digital Network (ISDN) (ISDN Digital Subscriber Line, IDSL), Single-pair High-bit-rate Digital Subscriber Line (SHDSL), Asymmetrical Digital Subscriber Line 2 (ADSL2), Asymmetrical Digital Subscriber Line 2plus (ADSL2plus), and Very-high-bit-rate Digital Subscriber Line 2 (VDSL2), etc.
In various digital subscriber line technologies (xDSL), except for baseband transmission DSL like IDSL and SHDSL, the DSL using pass band transmission allows DSL and Plain Old Telephone Service (POTS) to coexist in the same twist pair, by using frequency division multiplexing technology, with DSL occupying high frequency band and POTS occupying baseband portion below 4 KHz. POTS signal splits or integrates with DSL signal by Splitter/integrator.
For the twist pair used by DSL, the pairs are unshielded from each other. The electromagnetic interference between the pairs may result in the signal of one twist pair impacting that of another, i.e. crosstalk interference. With the development of DSL technology, the rate is becoming higher and higher and the frequency spectrum is becoming wider and wider. In new generation DSL technology VDSL2, a maximum bidirectional line rate reaches 200 Mbps and the frequency spectrum range reaches 40 MHz. Such a wide frequency spectrum range causes VDSL2 quite sensitive to the noise coupled from the crosstalk interference between cables, and Radio Frequency Interference (RFI). Situations like high bit error rate and high dropping rate may occur in practical application.
Virtual Noise scheme is one of the methods to improve the line stability. A transmitter-reference virtual noise (TXREFVN) is pre-configured in DSL transmitter end and is transferred to a receiver end during DSL training. In the training process, the receiver end may obtain an attenuation feature H log (f) of each subcarrier and calculate reference virtual noise at the receiver end. The receiver end selects a maximum value from an receiver-end-actual noise obtained in the training process and the receiver-end-reference virtual noise, and decides an assigned bit and gain on each subcarrier, in order to ensure, that after DSL line is activated, each subcarrier has a relatively high noise margin, and can resist the noise interference that might occur, therefore improving the stability of the line.
However, in the process of implementing the invention, the inventor finds that in the prior art the virtual noise is set manually according to a predicted possible maximum crosstalk situation. The set value may not comply with an actual operational situation and the virtual noise may be too high or too low, with the set value too high resulting in a high cost leading to performance loss, and the set value too low resulting in no protection to lines when the actual noise occurs.